From 8126b1c73108bc691f5643df19071a59a69d0bc6 Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Mon, 14 Mar 2022 19:59:53 +0100 Subject: pstore: Don't use semaphores in always-atomic-context code pstore_dump() is *always* invoked in atomic context (nowadays in an RCU read-side critical section, before that under a spinlock). It doesn't make sense to try to use semaphores here. This is mostly a revert of commit ea84b580b955 ("pstore: Convert buf_lock to semaphore"), except that two parts aren't restored back exactly as they were: - keep the lock initialization in pstore_register - in efi_pstore_write(), always set the "block" flag to false - omit "is_locked", that was unnecessary since commit 959217c84c27 ("pstore: Actually give up during locking failure") - fix the bailout message The actual problem that the buggy commit was trying to address may have been that the use of preemptible() in efi_pstore_write() was wrong - it only looks at preempt_count() and the state of IRQs, but __rcu_read_lock() doesn't touch either of those under CONFIG_PREEMPT_RCU. (Sidenote: CONFIG_PREEMPT_RCU means that the scheduler can preempt tasks in RCU read-side critical sections, but you're not allowed to actively block/reschedule.) Lockdep probably never caught the problem because it's very rare that you actually hit the contended case, so lockdep always just sees the down_trylock(), not the down_interruptible(), and so it can't tell that there's a problem. Fixes: ea84b580b955 ("pstore: Convert buf_lock to semaphore") Cc: stable@vger.kernel.org Acked-by: Sebastian Andrzej Siewior Signed-off-by: Jann Horn Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20220314185953.2068993-1-jannh@google.com --- fs/pstore/platform.c | 38 ++++++++++++++++++-------------------- 1 file changed, 18 insertions(+), 20 deletions(-) (limited to 'fs/pstore') diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c index f243cb5e6a4f..e26162f102ff 100644 --- a/fs/pstore/platform.c +++ b/fs/pstore/platform.c @@ -143,21 +143,22 @@ static void pstore_timer_kick(void) mod_timer(&pstore_timer, jiffies + msecs_to_jiffies(pstore_update_ms)); } -/* - * Should pstore_dump() wait for a concurrent pstore_dump()? If - * not, the current pstore_dump() will report a failure to dump - * and return. - */ -static bool pstore_cannot_wait(enum kmsg_dump_reason reason) +static bool pstore_cannot_block_path(enum kmsg_dump_reason reason) { - /* In NMI path, pstore shouldn't block regardless of reason. */ + /* + * In case of NMI path, pstore shouldn't be blocked + * regardless of reason. + */ if (in_nmi()) return true; switch (reason) { /* In panic case, other cpus are stopped by smp_send_stop(). */ case KMSG_DUMP_PANIC: - /* Emergency restart shouldn't be blocked. */ + /* + * Emergency restart shouldn't be blocked by spinning on + * pstore_info::buf_lock. + */ case KMSG_DUMP_EMERG: return true; default: @@ -389,21 +390,19 @@ static void pstore_dump(struct kmsg_dumper *dumper, unsigned long total = 0; const char *why; unsigned int part = 1; + unsigned long flags = 0; int ret; why = kmsg_dump_reason_str(reason); - if (down_trylock(&psinfo->buf_lock)) { - /* Failed to acquire lock: give up if we cannot wait. */ - if (pstore_cannot_wait(reason)) { - pr_err("dump skipped in %s path: may corrupt error record\n", - in_nmi() ? "NMI" : why); - return; - } - if (down_interruptible(&psinfo->buf_lock)) { - pr_err("could not grab semaphore?!\n"); + if (pstore_cannot_block_path(reason)) { + if (!spin_trylock_irqsave(&psinfo->buf_lock, flags)) { + pr_err("dump skipped in %s path because of concurrent dump\n", + in_nmi() ? "NMI" : why); return; } + } else { + spin_lock_irqsave(&psinfo->buf_lock, flags); } kmsg_dump_rewind(&iter); @@ -467,8 +466,7 @@ static void pstore_dump(struct kmsg_dumper *dumper, total += record.size; part++; } - - up(&psinfo->buf_lock); + spin_unlock_irqrestore(&psinfo->buf_lock, flags); } static struct kmsg_dumper pstore_dumper = { @@ -594,7 +592,7 @@ int pstore_register(struct pstore_info *psi) psi->write_user = pstore_write_user_compat; psinfo = psi; mutex_init(&psinfo->read_mutex); - sema_init(&psinfo->buf_lock, 1); + spin_lock_init(&psinfo->buf_lock); if (psi->flags & PSTORE_FLAGS_DMESG) allocate_buf_for_compression(); -- cgit v1.2.3