Hello. Ilia Mirkin wrote: > > Commit 040a0a37 "mutex: Add support for wound/wait style locks" used > > "!__builtin_constant_p(p == NULL)" which I guess the author meant that > > "__builtin_constant_p(p) && p", but gcc 3.x cannot handle such expression > > correctly, leading to boot failure when built with CONFIG_DEBUG_MUTEXES=y. > > I think that !__builtin_constant_p(p == NULL) is basically saying "I > am unable to conclude that p == NULL at build time", which would > translate to something along the lines of > > (__builtin_constant_p(p) && p) || !__builtin_constant_p(p) >
I think (__builtin_constant_p(p) && p) && p->acquired > 0 is safe but (!__builtin_constant_p(p)) && p->acquired > 0 is not safe, for "p != NULL" check is required for avoiding NULL pointer dereference. It seems to me that (!__builtin_constant_p(p == NULL)) need to be translated to something along the lines of (__builtin_constant_p(p) && p) || (!__builtin_constant_p(p) && p) which can be simplified as (p) . > Or perhaps it's just equivalent to !__builtin_constant_p(p), since the > compiler's ability to conclude whether it is NULL at build-time should > be unaffected by whether it actually is NULL or not. Likewise, it seems to me that (!__builtin_constant_p(p == NULL)) need to be translated to something along the lines of (!__builtin_constant_p(p) && p) . Well this change as well can fix "boot failure on gcc 3.x" and avoid "locking selftests failure on gcc 3.x / 4.x". OK, let's wait for answer from the author. Can I add "Signed-off-by: Ilia Mirkin <[email protected]>" to below patch? ---------- good patch start ---------- >From a8bbf6b3c2d44cb90d63820f146aaff119d871c9 Mon Sep 17 00:00:00 2001 From: Tetsuo Handa <[email protected]> Date: Sun, 8 Sep 2013 16:09:27 +0900 Subject: [PATCH] mutex: Avoid gcc version dependent __builtin_constant_p() usage. Commit 040a0a37 "mutex: Add support for wound/wait style locks" used "!__builtin_constant_p(p == NULL)" but gcc 3.x cannot handle such expression correctly, leading to boot failure when built with CONFIG_DEBUG_MUTEXES=y. Fix it by changing from "!__builtin_constant_p(p == NULL)" to "!__builtin_constant_p(p) && p". Signed-off-by: Tetsuo Handa <[email protected]> Cc: <[email protected]> [3.11+] --- kernel/mutex.c | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/mutex.c b/kernel/mutex.c index a52ee7bb..ef02003 100644 --- a/kernel/mutex.c +++ b/kernel/mutex.c @@ -448,7 +448,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, struct task_struct *owner; struct mspin_node node; - if (!__builtin_constant_p(ww_ctx == NULL) && ww_ctx->acquired > 0) { + if (!__builtin_constant_p(ww_ctx) && ww_ctx && ww_ctx->acquired > 0) { struct ww_mutex *ww; ww = container_of(lock, struct ww_mutex, base); @@ -478,7 +478,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, if ((atomic_read(&lock->count) == 1) && (atomic_cmpxchg(&lock->count, 1, 0) == 1)) { lock_acquired(&lock->dep_map, ip); - if (!__builtin_constant_p(ww_ctx == NULL)) { + if (!__builtin_constant_p(ww_ctx) && ww_ctx) { struct ww_mutex *ww; ww = container_of(lock, struct ww_mutex, base); @@ -548,7 +548,7 @@ slowpath: goto err; } - if (!__builtin_constant_p(ww_ctx == NULL) && ww_ctx->acquired > 0) { + if (!__builtin_constant_p(ww_ctx) && ww_ctx && ww_ctx->acquired > 0) { ret = __mutex_lock_check_stamp(lock, ww_ctx); if (ret) goto err; @@ -568,7 +568,7 @@ done: mutex_remove_waiter(lock, &waiter, current_thread_info()); mutex_set_owner(lock); - if (!__builtin_constant_p(ww_ctx == NULL)) { + if (!__builtin_constant_p(ww_ctx) && ww_ctx) { struct ww_mutex *ww = container_of(lock, struct ww_mutex, base); -- 1.7.8 ---------- good patch end ---------- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
