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Date: Fri, 21 May 2010 19:38:46 -0700
From: Salman <sqazi@...gle.com>
To: peterz@...radead.org, akpm@...ux-foundation.org,
mingo@...radead.org, linux-kernel@...r.kernel.org
Subject: [PATCH] Introduce a config option that introduces a bias in favour of
writers in rwlocks
If one or more readers are holding the lock, and one or more writers
are contending for it, then do not admit any new readers. However,
if a writer is holding a lock, then let readers contend for it at
equal footing with the writers.
This fixes a pathological case (see the code below), where the
tasklist_lock is continuously held by the readers, and the writers starve.
The change does not introduce any unexpected test failures in the locking
self-test. Furthermore, it makes the original problem go away. In
particular, after the change, the following code can run without
causing a lockup:
void *thread_code(void *args)
{
int j;
int pid2;
for (j = 0; j < 1000; j++) {
pid2 = fork();
if (pid2 == 0)
while(1) { sleep(1000); }
}
while (1) {
int status;
if (waitpid(-1, &status, WNOHANG)) {
printf("! %d\n", errno);
}
}
exit(0);
}
/*
* non-blocking waitpids in tight loop, with many children to go through,
* done on multiple thread, so that they can "pass the torch" to eachother
* and eliminate the window that a writer has to get in.
*
* This maximizes the holding of the tasklist_lock in read mode, starving
* any attempts to take the lock in the write mode.
*/
int main(int argc, char **argv)
{
int i;
pthread_attr_t attr;
pthread_t threads[NUM_CPUS];
for (i = 0; i < NUM_CPUS; i++) {
assert(!pthread_attr_init(&attr));
assert(!pthread_create(&threads[i], &attr, thread_code));
}
while(1) { sleep(1000);}
return 0;
}
Signed-off-by: "Salman Qazi" <sqazi@...gle.com>
---
arch/x86/include/asm/spinlock.h | 29 ++++++++++++++++++++++++++++-
arch/x86/include/asm/spinlock_types.h | 9 ++++++++-
lib/Kconfig | 11 +++++++++++
3 files changed, 47 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 3089f70..e05045b 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -234,7 +234,11 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
*/
static inline int arch_read_can_lock(arch_rwlock_t *lock)
{
+#ifdef CONFIG_RWLOCK_WRBIAS
+ return ((int)(lock)->lock > 0) && (!(lock)->writers_contending);
+#else
return (int)(lock)->lock > 0;
+#endif
}
/**
@@ -248,7 +252,15 @@ static inline int arch_write_can_lock(arch_rwlock_t *lock)
static inline void arch_read_lock(arch_rwlock_t *rw)
{
- asm volatile(LOCK_PREFIX " subl $1,(%0)\n\t"
+ asm volatile(
+#ifdef CONFIG_RWLOCK_WRBIAS
+ "cmpl $0, (%0)\n\t"
+ "jbe 3f\n\t"
+ "2: pause\n\t"
+ "cmpl $0, 0x04(%0)\n\t"
+ "jne 2b\n\t"
+#endif
+ "3 :" LOCK_PREFIX " subl $1,(%0)\n\t"
"jns 1f\n"
"call __read_lock_failed\n\t"
"1:\n"
@@ -259,7 +271,13 @@ static inline void arch_write_lock(arch_rwlock_t *rw)
{
asm volatile(LOCK_PREFIX " subl %1,(%0)\n\t"
"jz 1f\n"
+#ifdef CONFIG_RWLOCK_WRBIAS
+ LOCK_PREFIX " incl 0x04(%0)\n\t"
+#endif
"call __write_lock_failed\n\t"
+#ifdef CONFIG_RWLOCK_WRBIAS
+ LOCK_PREFIX " decl 0x04(%0)\n\t"
+#endif
"1:\n"
::LOCK_PTR_REG (rw), "i" (RW_LOCK_BIAS) : "memory");
}
@@ -268,6 +286,15 @@ static inline int arch_read_trylock(arch_rwlock_t *lock)
{
atomic_t *count = (atomic_t *)lock;
+#ifdef CONFIG_RWLOCK_WRBIAS
+
+ /* If there are writers contending but a writer doesn't have the lock
+ * then we shouldn't take it.
+ */
+ if ((lock->writers_contending) && (atomic_read(count) > 0))
+ return 0;
+#endif
+
if (atomic_dec_return(count) >= 0)
return 1;
atomic_inc(count);
diff --git a/arch/x86/include/asm/spinlock_types.h b/arch/x86/include/asm/spinlock_types.h
index dcb48b2..22908be 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -13,8 +13,15 @@ typedef struct arch_spinlock {
typedef struct {
unsigned int lock;
+#ifdef CONFIG_RWLOCK_WRBIAS
+ unsigned int writers_contending;
+#endif
} arch_rwlock_t;
-#define __ARCH_RW_LOCK_UNLOCKED { RW_LOCK_BIAS }
+#ifdef CONFIG_RWLOCK_WRBIAS
+ #define __ARCH_RW_LOCK_UNLOCKED { RW_LOCK_BIAS, 0 }
+#else
+ #define __ARCH_RW_LOCK_UNLOCKED { RW_LOCK_BIAS }
+#endif
#endif /* _ASM_X86_SPINLOCK_TYPES_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index 170d8ca..ca1cf79 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -210,4 +210,15 @@ config GENERIC_ATOMIC64
config LRU_CACHE
tristate
+config RWLOCK_WRBIAS
+ boolean
+ help
+ Introduce a config option that introduces a bias in favour of writers
+ in the readers-writer lock as follows:
+ if one or more readers are holding the lock, and one or more writers
+ are contending for it, then do not admit any new readers. However,
+ if a writer is holding a lock, then let readers contend for it at
+ equal footing with the writers.
+ default y
+
endmenu
--
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