[PATCH 07/11] nptl: Remove CANCELING_BITMASK
Adhemerval Zanella
adhemerval.zanella@linaro.org
Wed May 26 16:57:24 GMT 2021
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 is incurs in the same
issues described on BZ#12683: the cancellation is acting even *after*
the syscalls returns with user visible side-effects.
This patch removes this optimization and simplifies the pthread
cancellation implementation: pthread_cancel now first check if
cancellation is already pending and if not always send 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 alows 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.
---
nptl/cancellation.c | 12 ----
nptl/descr.h | 3 -
nptl/pthread_cancel.c | 124 ++++++++++---------------------------
nptl/pthread_join_common.c | 2 +-
4 files changed, 35 insertions(+), 106 deletions(-)
diff --git a/nptl/cancellation.c b/nptl/cancellation.c
index c20845adc0..b15f25d8f6 100644
--- a/nptl/cancellation.c
+++ b/nptl/cancellation.c
@@ -88,17 +88,5 @@ __pthread_disable_asynccancel (int oldtype)
/* Prepare the next round. */
oldval = curval;
}
-
- /* We cannot return when we are being canceled. Upon return the
- thread might be things which would have to be undone. The
- following loop should loop until the cancellation signal is
- delivered. */
- while (__builtin_expect ((newval & (CANCELING_BITMASK | CANCELED_BITMASK))
- == CANCELING_BITMASK, 0))
- {
- futex_wait_simple ((unsigned int *) &self->cancelhandling, newval,
- FUTEX_PRIVATE);
- newval = THREAD_GETMEM (self, cancelhandling);
- }
}
libc_hidden_def (__pthread_disable_asynccancel)
diff --git a/nptl/descr.h b/nptl/descr.h
index 9d8297b45f..a120365f88 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -283,9 +283,6 @@ struct pthread
/* Bit set if asynchronous cancellation mode is selected. */
#define CANCELTYPE_BIT 1
#define CANCELTYPE_BITMASK (0x01 << CANCELTYPE_BIT)
- /* Bit set if canceling has been initiated. */
-#define CANCELING_BIT 2
-#define CANCELING_BITMASK (0x01 << CANCELING_BIT)
/* Bit set if canceled. */
#define CANCELED_BIT 3
#define CANCELED_BITMASK (0x01 << CANCELED_BIT)
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index f264a4096a..c11a2b4551 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -43,35 +43,18 @@ sigcancel_handler (int sig, siginfo_t *si, void *ctx)
struct pthread *self = THREAD_SELF;
- int oldval = THREAD_GETMEM (self, cancelhandling);
- while (1)
- {
- /* We are canceled now. When canceled by another thread this flag
- is already set but if the signal is directly send (internally or
- from another process) is has to be done here. */
- int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
-
- if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
- /* Already canceled or exiting. */
- break;
-
- int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
- oldval);
- if (curval == oldval)
- {
- /* Set the return value. */
- THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-
- /* Make sure asynchronous cancellation is still enabled. */
- if ((newval & CANCELTYPE_BITMASK) != 0)
- /* Run the registered destructors and terminate the thread. */
- __do_cancel ();
-
- break;
- }
-
- oldval = curval;
- }
+ int ch = atomic_load_relaxed (&self->cancelhandling);
+ /* Cancelation not enabled, not cancelled, or already exitting. */
+ if ((ch & CANCELSTATE_BITMASK) != 0
+ || (ch & CANCELED_BITMASK) == 0
+ || (ch & EXITING_BITMASK) != 0)
+ return;
+
+ /* Set the return value. */
+ THREAD_SETMEM (self, result, PTHREAD_CANCELED);
+ /* Make sure asynchronous cancellation is still enabled. */
+ if ((ch & CANCELTYPE_BITMASK) != 0)
+ __do_cancel ();
}
int
@@ -104,72 +87,33 @@ __pthread_cancel (pthread_t th)
" must be installed for pthread_cancel to work\n");
}
#endif
- int result = 0;
- int oldval;
- int newval;
- do
+
+ int oldch = atomic_fetch_or_acquire (&pd->cancelhandling, CANCELED_BITMASK);
+ if ((oldch & CANCELED_BITMASK) != 0)
+ return 0;
+
+ if (pd == THREAD_SELF)
{
- again:
- oldval = pd->cancelhandling;
- newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
-
- /* Avoid doing unnecessary work. The atomic operation can
- potentially be expensive if the bug has to be locked and
- remote cache lines have to be invalidated. */
- if (oldval == newval)
- break;
-
- /* If the cancellation is handled asynchronously just send a
- signal. We avoid this if possible since it's more
- expensive. */
- if (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
- {
- /* Mark the cancellation as "in progress". */
- if (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling,
- oldval | CANCELING_BITMASK,
- oldval))
- goto again;
-
- if (pd == THREAD_SELF)
- /* This is not merely an optimization: An application may
- call pthread_cancel (pthread_self ()) without calling
- pthread_create, so the signal handler may not have been
- set up for a self-cancel. */
- {
- THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
- if ((newval & CANCELTYPE_BITMASK) != 0)
- __do_cancel ();
- }
- else
- {
- /* The cancellation handler will take care of marking the
- thread as canceled. */
- pid_t pid = __getpid ();
-
- int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid,
- SIGCANCEL);
- if (INTERNAL_SYSCALL_ERROR_P (val))
- result = INTERNAL_SYSCALL_ERRNO (val);
- }
-
- break;
- }
-
- /* A single-threaded process should be able to kill itself, since
- there is nothing in the POSIX specification that says that it
- cannot. So we set multiple_threads to true so that cancellation
- points get executed. */
- THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
+ /* A single-threaded process should be able to kill itself, since there
+ is nothing in the POSIX specification that says that it cannot. So
+ we set multiple_threads to true so that cancellation points get
+ executed. */
+ THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
#ifndef TLS_MULTIPLE_THREADS_IN_TCB
- __libc_multiple_threads = 1;
+ __libc_multiple_threads = 1;
#endif
+
+ THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
+ if ((oldch & CANCELTYPE_BITMASK) != 0)
+ __do_cancel ();
+ return 0;
}
- /* Mark the thread as canceled. This has to be done
- atomically since other bits could be modified as well. */
- while (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling, newval,
- oldval));
- return result;
+ pid_t pid = __getpid ();
+ int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid, SIGCANCEL);
+ return INTERNAL_SYSCALL_ERROR_P (val)
+ ? INTERNAL_SYSCALL_ERRNO (val)
+ : 0;
}
versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);
diff --git a/nptl/pthread_join_common.c b/nptl/pthread_join_common.c
index e87801b5a3..f842c91a08 100644
--- a/nptl/pthread_join_common.c
+++ b/nptl/pthread_join_common.c
@@ -57,7 +57,7 @@ __pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
if ((pd == self
|| (self->joinid == pd
&& (pd->cancelhandling
- & (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
+ & (CANCELED_BITMASK | EXITING_BITMASK
| TERMINATED_BITMASK)) == 0))
&& !CANCEL_ENABLED_AND_CANCELED (self->cancelhandling))
/* This is a deadlock situation. The threads are waiting for each
--
2.30.2
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