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commit fb14b54b83f3a5517c249f86f34dfcd4ffe0202c
Author: Xiang Xiao <xiaoxi...@xiaomi.com>
AuthorDate: Sun Jun 22 03:03:59 2025 +0800
libc/semaphore: Use the while loop to handle the inerrupt correctly
If atomic_try_cmpxchg_xxxx runs on LL/SC architectures (e.g.ARMv7,
ARMv8, RISC-V), the weak CAS expands to a single LDREX/STREX pair.
If the CPU takes an IRQ/FIQ/SVC between the two instructions,
hardware performs an implicit CLREX and the following STREX returns
1, therefore atomic_try_cmpxchg_xxxx return failure even though
*addr* still holds the expected value.
So let's retry atomic_try_cmpxchg_xxxx in this case.
Signed-off-by: Xiang Xiao <xiaoxi...@xiaomi.com>
---
libs/libc/semaphore/sem_post.c | 19 +++++++++----------
libs/libc/semaphore/sem_trywait.c | 40 ++++++++++++++++-----------------------
libs/libc/semaphore/sem_wait.c | 19 +++++++++----------
3 files changed, 34 insertions(+), 44 deletions(-)
diff --git a/libs/libc/semaphore/sem_post.c b/libs/libc/semaphore/sem_post.c
index 874954a8a21..d34b6fea566 100644
--- a/libs/libc/semaphore/sem_post.c
+++ b/libs/libc/semaphore/sem_post.c
@@ -151,24 +151,26 @@ int nxsem_post(FAR sem_t *sem)
}
# endif
- if (fastpath)
+ while (fastpath)
{
- int32_t old;
- int32_t new;
FAR atomic_t *val = mutex ? NXSEM_MHOLDER(sem) : NXSEM_COUNT(sem);
+ int32_t old = atomic_read(val);
+ int32_t new;
if (mutex)
{
- old = _SCHED_GETTID();
+ if (NXSEM_MBLOCKING(old))
+ {
+ break;
+ }
+
new = NXSEM_NO_MHOLDER;
}
else
{
- old = atomic_read(val);
-
if (old < 0)
{
- goto out;
+ break;
}
new = old + 1;
@@ -179,9 +181,6 @@ int nxsem_post(FAR sem_t *sem)
return OK;
}
}
-
-out:
-
#else
UNUSED(mutex);
UNUSED(fastpath);
diff --git a/libs/libc/semaphore/sem_trywait.c
b/libs/libc/semaphore/sem_trywait.c
index 39112aa219d..8f1686ea023 100644
--- a/libs/libc/semaphore/sem_trywait.c
+++ b/libs/libc/semaphore/sem_trywait.c
@@ -146,43 +146,35 @@ int nxsem_trywait(FAR sem_t *sem)
}
# endif
- if (fastpath)
+ while (fastpath)
{
- bool ret = false;
- int32_t old;
- int32_t new;
FAR atomic_t *val = mutex ? NXSEM_MHOLDER(sem) : NXSEM_COUNT(sem);
+ int32_t old = atomic_read(val);
+ int32_t new;
if (mutex)
{
- old = NXSEM_NO_MHOLDER;
+ if (old != NXSEM_NO_MHOLDER)
+ {
+ return -EAGAIN;
+ }
+
+ new = _SCHED_GETTID();
}
else
{
- old = atomic_read(val);
- }
-
- do
- {
- if (!mutex)
- {
- if (old < 1)
- {
- break;
- }
-
- new = old - 1;
- }
- else
+ if (old < 1)
{
- new = _SCHED_GETTID();
+ return -EAGAIN;
}
- ret = atomic_try_cmpxchg_acquire(NXSEM_MHOLDER(sem), &old, new);
+ new = old - 1;
}
- while (!mutex && !ret);
- return ret ? OK : -EAGAIN;
+ if (atomic_try_cmpxchg_acquire(val, &old, new))
+ {
+ return OK;
+ }
}
#else
diff --git a/libs/libc/semaphore/sem_wait.c b/libs/libc/semaphore/sem_wait.c
index 6a187e23acd..dca20eb4bd6 100644
--- a/libs/libc/semaphore/sem_wait.c
+++ b/libs/libc/semaphore/sem_wait.c
@@ -174,24 +174,26 @@ int nxsem_wait(FAR sem_t *sem)
}
# endif
- if (fastpath)
+ while (fastpath)
{
- int32_t old;
- int32_t new;
FAR atomic_t *val = mutex ? NXSEM_MHOLDER(sem) : NXSEM_COUNT(sem);
+ int32_t old = atomic_read(val);
+ int32_t new;
if (mutex)
{
- old = NXSEM_NO_MHOLDER;
+ if (old != NXSEM_NO_MHOLDER)
+ {
+ break;
+ }
+
new = _SCHED_GETTID();
}
else
{
- old = atomic_read(val);
-
if (old < 1)
{
- goto out;
+ break;
}
new = old - 1;
@@ -202,9 +204,6 @@ int nxsem_wait(FAR sem_t *sem)
return OK;
}
}
-
-out:
-
#else
UNUSED(mutex);
UNUSED(fastpath);
