nptl: Remove CANCELING_BITMASK
The CANCELING_BITMASK is used as an optimization to avoid sending the signal when pthread_cancel is called in a concurrent manner. This requires then to put both the cancellation state and type on a shared state (cancelhandling), since 'pthread_cancel' checks whether cancellation is enabled and asynchrnous to either cancel itself of sending the signal. It also requires handle the CANCELING_BITMASK on __pthread_disable_asynccancel, however this incurs in the same issues described on BZ#12683: the cancellation is acted upon even *after* syscall returns with user visible side-effects. This patch removes this optimization and simplifies the pthread cancellation implementation: pthread_cancel now first checks if cancellation is already pending and if not always, sends a signal if the target is not itself. The SIGCANCEL handler is also simpified since there is not need to setup a CAS loop. It also allows to move both the cancellation state and mode out of 'cancelhadling' (it is done in subsequent patches). Checked on x86_64-linux-gnu and aarch64-linux-gnu.
This commit is contained in:
Adhemerval Zanella
parent
41c7295617
commit
26cfbb7162
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@ -88,17 +88,5 @@ __pthread_disable_asynccancel (int oldtype)
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/* Prepare the next round. */
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oldval = curval;
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}
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/* We cannot return when we are being canceled. Upon return the
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thread might be things which would have to be undone. The
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following loop should loop until the cancellation signal is
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delivered. */
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while (__builtin_expect ((newval & (CANCELING_BITMASK | CANCELED_BITMASK))
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== CANCELING_BITMASK, 0))
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{
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futex_wait_simple ((unsigned int *) &self->cancelhandling, newval,
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FUTEX_PRIVATE);
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newval = THREAD_GETMEM (self, cancelhandling);
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}
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}
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libc_hidden_def (__pthread_disable_asynccancel)
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@ -283,9 +283,6 @@ struct pthread
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/* Bit set if asynchronous cancellation mode is selected. */
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#define CANCELTYPE_BIT 1
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#define CANCELTYPE_BITMASK (0x01 << CANCELTYPE_BIT)
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/* Bit set if canceling has been initiated. */
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#define CANCELING_BIT 2
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#define CANCELING_BITMASK (0x01 << CANCELING_BIT)
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/* Bit set if canceled. */
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#define CANCELED_BIT 3
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#define CANCELED_BITMASK (0x01 << CANCELED_BIT)
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@ -43,35 +43,18 @@ sigcancel_handler (int sig, siginfo_t *si, void *ctx)
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struct pthread *self = THREAD_SELF;
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int oldval = THREAD_GETMEM (self, cancelhandling);
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while (1)
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{
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/* We are canceled now. When canceled by another thread this flag
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is already set but if the signal is directly send (internally or
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from another process) is has to be done here. */
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int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
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int ch = atomic_load_relaxed (&self->cancelhandling);
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/* Cancelation not enabled, not cancelled, or already exitting. */
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if ((ch & CANCELSTATE_BITMASK) != 0
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|| (ch & CANCELED_BITMASK) == 0
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|| (ch & EXITING_BITMASK) != 0)
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return;
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if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
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/* Already canceled or exiting. */
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break;
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int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
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oldval);
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if (curval == oldval)
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{
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/* Set the return value. */
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THREAD_SETMEM (self, result, PTHREAD_CANCELED);
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/* Make sure asynchronous cancellation is still enabled. */
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if ((newval & CANCELTYPE_BITMASK) != 0)
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/* Run the registered destructors and terminate the thread. */
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__do_cancel ();
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break;
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}
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oldval = curval;
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}
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/* Set the return value. */
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THREAD_SETMEM (self, result, PTHREAD_CANCELED);
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/* Make sure asynchronous cancellation is still enabled. */
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if ((ch & CANCELTYPE_BITMASK) != 0)
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__do_cancel ();
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}
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int
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@ -104,72 +87,34 @@ __pthread_cancel (pthread_t th)
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" must be installed for pthread_cancel to work\n");
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}
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#endif
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int result = 0;
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int oldval;
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int newval;
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do
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int oldch = atomic_fetch_or_acquire (&pd->cancelhandling, CANCELED_BITMASK);
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if ((oldch & CANCELED_BITMASK) != 0)
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return 0;
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if (pd == THREAD_SELF)
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{
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again:
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oldval = pd->cancelhandling;
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newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
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/* Avoid doing unnecessary work. The atomic operation can
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potentially be expensive if the bug has to be locked and
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remote cache lines have to be invalidated. */
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if (oldval == newval)
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break;
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/* If the cancellation is handled asynchronously just send a
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signal. We avoid this if possible since it's more
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expensive. */
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if (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
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{
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/* Mark the cancellation as "in progress". */
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if (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling,
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oldval | CANCELING_BITMASK,
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oldval))
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goto again;
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if (pd == THREAD_SELF)
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/* This is not merely an optimization: An application may
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call pthread_cancel (pthread_self ()) without calling
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pthread_create, so the signal handler may not have been
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set up for a self-cancel. */
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{
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THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
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if ((newval & CANCELTYPE_BITMASK) != 0)
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__do_cancel ();
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}
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else
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{
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/* The cancellation handler will take care of marking the
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thread as canceled. */
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pid_t pid = __getpid ();
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int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid,
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SIGCANCEL);
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if (INTERNAL_SYSCALL_ERROR_P (val))
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result = INTERNAL_SYSCALL_ERRNO (val);
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}
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break;
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}
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/* A single-threaded process should be able to kill itself, since
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there is nothing in the POSIX specification that says that it
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cannot. So we set multiple_threads to true so that cancellation
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points get executed. */
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THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
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/* A single-threaded process should be able to kill itself, since there
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is nothing in the POSIX specification that says that it cannot. So
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we set multiple_threads to true so that cancellation points get
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executed. */
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THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
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#ifndef TLS_MULTIPLE_THREADS_IN_TCB
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__libc_multiple_threads = 1;
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__libc_multiple_threads = 1;
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#endif
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}
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/* Mark the thread as canceled. This has to be done
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atomically since other bits could be modified as well. */
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while (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling, newval,
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oldval));
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return result;
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THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
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if ((oldch & CANCELSTATE_BITMASK) == 0
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&& (oldch & CANCELTYPE_BITMASK) != 0)
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__do_cancel ();
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return 0;
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}
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pid_t pid = __getpid ();
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int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid, SIGCANCEL);
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return INTERNAL_SYSCALL_ERROR_P (val)
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? INTERNAL_SYSCALL_ERRNO (val)
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: 0;
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}
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versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);
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@ -57,7 +57,7 @@ __pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
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if ((pd == self
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|| (self->joinid == pd
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&& (pd->cancelhandling
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& (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
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& (CANCELED_BITMASK | EXITING_BITMASK
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| TERMINATED_BITMASK)) == 0))
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&& !CANCEL_ENABLED_AND_CANCELED (self->cancelhandling))
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/* This is a deadlock situation. The threads are waiting for each
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