[PATCH v2] mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock

Tetsuo Handa
2023-04-04T14:32:19Z

syzbot is reporting circular locking dependency which involves
zonelist_update_seq seqlock [1], for this lock is checked by memory
allocation requests which do not need to be retried.

One deadlock scenario is kmalloc(GFP_ATOMIC) from an interrupt handler.

CPU0
----
__build_all_zonelists() {
write_seqlock(&zonelist_update_seq); // makes zonelist_update_seq.seqcount odd
// e.g. timer interrupt handler runs at this moment
some_timer_func() {
kmalloc(GFP_ATOMIC) {
__alloc_pages_slowpath() {
read_seqbegin(&zonelist_update_seq) {
// spins forever because zonelist_update_seq.seqcount is odd
}
}
}
}
// e.g. timer interrupt handler finishes
write_sequnlock(&zonelist_update_seq); // makes zonelist_update_seq.seqcount even
}

This deadlock scenario can be easily eliminated by not calling
read_seqbegin(&zonelist_update_seq) from !__GFP_DIRECT_RECLAIM allocation
requests, for retry is applicable to only __GFP_DIRECT_RECLAIM allocation
requests. But Michal Hocko does not know whether we should go with this
approach.

Another deadlock scenario which syzbot is reporting is a race between
kmalloc(GFP_ATOMIC) from tty_insert_flip_string_and_push_buffer()
with port->lock held and printk() from __build_all_zonelists() with
zonelist_update_seq held.

CPU0 CPU1
---- ----
pty_write() {
tty_insert_flip_string_and_push_buffer() {
__build_all_zonelists() {
write_seqlock(&zonelist_update_seq);
build_zonelists() {
printk() {
vprintk() {
vprintk_default() {
vprintk_emit() {
console_unlock() {
console_flush_all() {
console_emit_next_record() {
con->write() = serial8250_console_write() {
spin_lock_irqsave(&port->lock, flags);
tty_insert_flip_string() {
tty_insert_flip_string_fixed_flag() {
__tty_buffer_request_room() {
tty_buffer_alloc() {
kmalloc(GFP_ATOMIC | __GFP_NOWARN) {
__alloc_pages_slowpath() {
zonelist_iter_begin() {
read_seqbegin(&zonelist_update_seq); // spins forever because zonelist_update_seq.seqcount is odd
spin_lock_irqsave(&port->lock, flags); // spins forever because port->lock is held
}
}
}
}
}
}
}
}
spin_unlock_irqrestore(&port->lock, flags);
// message is printed to console
spin_unlock_irqrestore(&port->lock, flags);
}
}
}
}
}
}
}
}
}
write_sequnlock(&zonelist_update_seq);
}
}
}

This deadlock scenario can be eliminated by

preventing interrupt context from calling kmalloc(GFP_ATOMIC)

and

preventing printk() from calling console_flush_all()

while zonelist_update_seq.seqcount is odd.

Since Petr Mladek thinks that __build_all_zonelists() can become a
candidate for deferring printk() [2], let's address this problem by

disabling local interrupts in order to avoid kmalloc(GFP_ATOMIC)

and

disabling synchronous printk() in order to avoid console_flush_all()

.

As a side effect of minimizing duration of zonelist_update_seq.seqcount
being odd by disabling synchronous printk(), latency at
read_seqbegin(&zonelist_update_seq) for both !__GFP_DIRECT_RECLAIM and
__GFP_DIRECT_RECLAIM allocation requests will be reduced. Although, from
lockdep perspective, not calling read_seqbegin(&zonelist_update_seq) (i.e.
do not record unnecessary locking dependency) from interrupt context is
still preferable, even if we don't allow calling kmalloc(GFP_ATOMIC) inside
write_seqlock(&zonelist_update_seq)/write_sequnlock(&zonelist_update_seq)
section...

Reported-by: syzbot <syzbot+223c7461c58c58a4cb10@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=223c7461c58c58a4cb10 [1]
Fixes: 3d36424b3b58 ("mm/page_alloc: fix race condition between build_all_zonelists and page allocation")
Link: https://lkml.kernel.org/r/ZCrs+1cDqPWTDFNM@alley [2]
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Petr Mladek <pmladek@suse.com>
---
Changes in v2:
Update patch description and comment.

mm/page_alloc.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7136c36c5d01..e8b4f294d763 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6632,7 +6632,21 @@ static void __build_all_zonelists(void *data)
int nid;
int __maybe_unused cpu;
pg_data_t *self = data;
+ unsigned long flags;

+ /*
+ * Explicitly disable this CPU's interrupts before taking seqlock
+ * to prevent any IRQ handler from calling into the page allocator
+ * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
+ */
+ local_irq_save(flags);
+ /*
+ * Explicitly disable this CPU's synchronous printk() before taking
+ * seqlock to prevent any printk() from trying to hold port->lock, for
+ * tty_insert_flip_string_and_push_buffer() on other CPU might be
+ * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
+ */
+ printk_deferred_enter();
write_seqlock(&zonelist_update_seq);

#ifdef CONFIG_NUMA
@@ -6671,6 +6685,8 @@ static void __build_all_zonelists(void *data)
}

write_sequnlock(&zonelist_update_seq);
+ printk_deferred_exit();
+ local_irq_restore(flags);
}

static noinline void __init
--
2.34.1