This is a note to let you know that I've just added the patch titled
printk: Defer legacy printing when holding printk_cpu_sync
to the 6.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
printk-defer-legacy-printing-when-holding-printk_cpu.patch
and it can be found in the queue-6.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit a5ba1301f49e51058910567a26f2c8d42d372092
Author: John Ogness <john.ogness@xxxxxxxxxxxxx>
Date: Mon Dec 9 12:23:46 2024 +0106
printk: Defer legacy printing when holding printk_cpu_sync
[ Upstream commit 0161e2d6950fe66cf6ac1c10d945bae971f33667 ]
The documentation of printk_cpu_sync_get() clearly states
that the owner must never perform any activities where it waits
for a CPU. For legacy printing there can be spinning on the
console_lock and on the port lock. Therefore legacy printing
must be deferred when holding the printk_cpu_sync.
Note that in the case of emergency states, atomic consoles
are not prevented from printing when printk is deferred. This
is appropriate because they do not spin-wait indefinitely for
other CPUs.
Reported-by: Rik van Riel <riel@xxxxxxxxxxx>
Closes: https://lore.kernel.org/r/20240715232052.73eb7fb1@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Fixes: 55d6af1d6688 ("lib/nmi_backtrace: explicitly serialize banner and regs")
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lore.kernel.org/r/20241209111746.192559-3-john.ogness@xxxxxxxxxxxxx
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 3fcb48502adbd..5eef70000b439 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -335,3 +335,9 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped);
void console_prepend_replay(struct printk_message *pmsg);
#endif
+
+#ifdef CONFIG_SMP
+bool is_printk_cpu_sync_owner(void);
+#else
+static inline bool is_printk_cpu_sync_owner(void) { return false; }
+#endif
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index beb808f4c367b..7530df62ff7cb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -4892,6 +4892,11 @@ void console_try_replay_all(void)
static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);
+bool is_printk_cpu_sync_owner(void)
+{
+ return (atomic_read(&printk_cpu_sync_owner) == raw_smp_processor_id());
+}
+
/**
* __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant
* spinning lock is not owned by any CPU.
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 2b35a9d3919d8..e6198da7c7354 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -43,10 +43,15 @@ bool is_printk_legacy_deferred(void)
/*
* The per-CPU variable @printk_context can be read safely in any
* context. CPU migration is always disabled when set.
+ *
+ * A context holding the printk_cpu_sync must not spin waiting for
+ * another CPU. For legacy printing, it could be the console_lock
+ * or the port lock.
*/
return (force_legacy_kthread() ||
this_cpu_read(printk_context) ||
- in_nmi());
+ in_nmi() ||
+ is_printk_cpu_sync_owner());
}
asmlinkage int vprintk(const char *fmt, va_list args)
