Messages in this thread Patch in this message | | | Date | Sun, 11 Nov 2012 19:27:44 +0100 | From | Oleg Nesterov <> | Subject | [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix |
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More include's and more comments, no changes in code.
To remind, once/if I am sure you agree with this patch I'll send 2 additional and simple patches:
1. lockdep annotations
2. CONFIG_PERCPU_RWSEM
It seems that we can do much more improvements to a) speedup the writers and b) make percpu_rw_semaphore more useful, but not right now.
Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- lib/percpu-rwsem.c | 35 +++++++++++++++++++++++++++++++++-- 1 files changed, 33 insertions(+), 2 deletions(-)
diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c index 0e3bc0f..02bd157 100644 --- a/lib/percpu-rwsem.c +++ b/lib/percpu-rwsem.c @@ -1,6 +1,11 @@ +#include <linux/mutex.h> +#include <linux/rwsem.h> +#include <linux/percpu.h> +#include <linux/wait.h> #include <linux/percpu-rwsem.h> #include <linux/rcupdate.h> #include <linux/sched.h> +#include <linux/errno.h> int percpu_init_rwsem(struct percpu_rw_semaphore *brw) { @@ -21,6 +26,29 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw) brw->fast_read_ctr = NULL; /* catch use after free bugs */ } +/* + * This is the fast-path for down_read/up_read, it only needs to ensure + * there is no pending writer (!mutex_is_locked() check) and inc/dec the + * fast per-cpu counter. The writer uses synchronize_sched() to serialize + * with the preempt-disabled section below. + * + * The nontrivial part is that we should guarantee acquire/release semantics + * in case when + * + * R_W: down_write() comes after up_read(), the writer should see all + * changes done by the reader + * or + * W_R: down_read() comes after up_write(), the reader should see all + * changes done by the writer + * + * If this helper fails the callers rely on the normal rw_semaphore and + * atomic_dec_and_test(), so in this case we have the necessary barriers. + * + * But if it succeeds we do not have any barriers, mutex_is_locked() or + * __this_cpu_add() below can be reordered with any LOAD/STORE done by the + * reader inside the critical section. See the comments in down_write and + * up_write below. + */ static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) { bool success = false; @@ -98,6 +126,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw) * * 3. Ensures that if any reader has exited its critical section via * fast-path, it executes a full memory barrier before we return. + * See R_W case in the comment above update_fast_ctr(). */ synchronize_sched(); @@ -116,8 +145,10 @@ void percpu_up_write(struct percpu_rw_semaphore *brw) /* allow the new readers, but only the slow-path */ up_write(&brw->rw_sem); - /* insert the barrier before the next fast-path in down_read */ + /* + * Insert the barrier before the next fast-path in down_read, + * see W_R case in the comment above update_fast_ctr(). + */ synchronize_sched(); - mutex_unlock(&brw->writer_mutex); } -- 1.5.5.1
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