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SubjectRe: [PATCH v6 1/6] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath
On Thu, Nov 17, 2022 at 09:20:11PM -0500, Waiman Long wrote:
> A non-first waiter can potentially spin in the for loop of
> rwsem_down_write_slowpath() without sleeping but fail to acquire the
> lock even if the rwsem is free if the following sequence happens:
>
> Non-first RT waiter First waiter Lock holder
> ------------------- ------------ -----------
> Acquire wait_lock
> rwsem_try_write_lock():
> Set handoff bit if RT or
> wait too long
> Set waiter->handoff_set
> Release wait_lock
> Acquire wait_lock
> Inherit waiter->handoff_set
> Release wait_lock
> Clear owner
> Release lock
> if (waiter.handoff_set) {
> rwsem_spin_on_owner(();
> if (OWNER_NULL)
> goto trylock_again;
> }
> trylock_again:
> Acquire wait_lock
> rwsem_try_write_lock():
> if (first->handoff_set && (waiter != first))
> return false;
> Release wait_lock
>
> A non-first waiter cannot really acquire the rwsem even if it mistakenly
> believes that it can spin on OWNER_NULL value. If that waiter happens
> to be an RT task running on the same CPU as the first waiter, it can
> block the first waiter from acquiring the rwsem leading to live lock.
> Fix this problem by making sure that a non-first waiter cannot spin in
> the slowpath loop without sleeping.
>
> Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent")
> Reviewed-and-tested-by: Mukesh Ojha <quic_mojha@quicinc.com>
> Signed-off-by: Waiman Long <longman@redhat.com>
> Cc: stable@vger.kernel.org
> ---

I was checking if commit 6eebd5fb2083 ("locking/rwsem: Allow slowpath writer to ignore handoff bit if not set by first waiter") resolves the issue that was discussed in [1]. I modified the program and script slighly:
fsim.c:
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
void sig_handle(int sig) { exit(0); }
int main(void)
{
unsigned long c;
signal(SIGALRM, sig_handle);
alarm(3);
while (1)
c++;
}

run-fsim.sh:
#!/bin/bash
if [ ! -e fsim ]; then
gcc -o fsim fsim.c
if [ $? -ne 0 ]; then
echo Failed to compile fsim
exit -1
fi
fi
MAX_ITERATIONS=20000
#The fsim processes are meant to run on both logical CPUs belonging to a CPU core, e.g. 1 and 129.
CPU_RANGE1="${1:-1 11}"
CPU_RANGE2="${1:-129 139}"
for i in `seq 1 $MAX_ITERATIONS`; do
echo "Start $i/$MAX_ITERATIONS: `date`"
for CPU in `seq $CPU_RANGE1` `seq $CPU_RANGE2`; do
taskset -c $CPU chrt -r 10 ./fsim &>/dev/null &
taskset -c $CPU chrt -r 20 ./fsim &>/dev/null &
taskset -c $CPU chrt -r 30 ./fsim &>/dev/null &
taskset -c $CPU chrt -r 40 ./fsim &>/dev/null &
done
echo "Wait $i/$MAX_ITERATIONS: `date`"
wait
done

No soft lockups were triggered but after 1.5 hours of testing, the fsim processes got stuck and only one of them was visible in the output of top:
> top - 18:45:01 up 44 min, 3 users, load average: 72.00, 71.04, 54.81
> Tasks: 2226 total, 4 running, 2222 sleeping, 0 stopped, 0 zombie
> %Cpu(s): 0.0 us, 0.4 sy, 0.0 ni, 99.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> MiB Mem : 239777.1+total, 235671.7+free, 4332.156 used, 1435.641 buff/cache
> MiB Swap: 1023.996 total, 1023.996 free, 0.000 used. 235444.9+avail Mem
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 100666 root -31 0 0 0 0 D 94.84 0.000 14:59.40 fsim
> 98193 root 20 0 42224 6844 3484 R 0.794 0.003 0:07.05 top
> 1 root 20 0 79220 12544 9124 S 0.000 0.005 0:11.95 systemd

All of the fsim processes got stuck at the same code path - while exiting:
> [ 2462.649033] INFO: task fsim:100600 blocked for more than 491 seconds.
> [ 2462.649036] Tainted: G E N 5.14.21-sle15-sp5-221214-hoff3-7 #8
> [ 2462.649038] task:fsim state:D stack: 0 pid:100600 ppid: 95456 flags:0x00000000
> [ 2462.649042] Call Trace:
> [ 2462.649045] <TASK>
> [ 2462.649046] __schedule+0x2cd/0x1140
> [ 2462.649059] schedule+0x5c/0xc0
> [ 2462.649061] rwsem_down_write_slowpath+0x349/0x5d0
> [ 2462.649070] unlink_file_vma+0x2d/0x60
> [ 2462.649074] free_pgtables+0x67/0x110
> [ 2462.649083] exit_mmap+0xaf/0x1f0
> [ 2462.649088] mmput+0x56/0x120
> [ 2462.649090] do_exit+0x306/0xb50
> [ 2462.649095] do_group_exit+0x3a/0xa0
> [ 2462.649098] __x64_sys_exit_group+0x14/0x20
> [ 2462.649102] do_syscall_64+0x5b/0x80
> [ 2462.649116] entry_SYSCALL_64_after_hwframe+0x61/0xcb
> [ 2462.649120] RIP: 0033:0x7f90abae6c46
> [ 2462.649122] RSP: 002b:00007ffc0ca21638 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> [ 2462.649124] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f90abae6c46
> [ 2462.649125] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
> [ 2462.649127] RBP: 00007f90abdf5970 R08: 00000000000000e7 R09: ffffffffffffff80
> [ 2462.649128] R10: 0000000000000002 R11: 0000000000000246 R12: 00007f90abdf5970
> [ 2462.649129] R13: 0000000000000001 R14: 00007f90abdf9328 R15: 0000000000000000
> [ 2462.649132] </TASK>
> [ 2462.649133] INFO: task fsim:100601 blocked for more than 491 seconds.
> [ 2462.649216] INFO: task fsim:100603 blocked for more than 491 seconds.
> [ 2462.649295] INFO: task fsim:100604 blocked for more than 491 seconds.
> [ 2462.649371] INFO: task fsim:100605 blocked for more than 491 seconds.
> [ 2462.649449] INFO: task fsim:100606 blocked for more than 491 seconds.
> [ 2462.649526] INFO: task fsim:100607 blocked for more than 491 seconds.
> [ 2462.649606] INFO: task fsim:100608 blocked for more than 491 seconds.
> [ 2462.649676] INFO: task fsim:100609 blocked for more than 491 seconds.

So, I tested these fixes all together added on top of 6eebd5fb2083:
* locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath
* locking/rwsem: Disable preemption at all down_read*() and up_read() code paths
* locking/rwsem: Disable preemption at all down_write*() and up_write() code paths

After 20 hours of runtime, none of the fsim processes got stuck nor any soft lockups occurred. AFAICT, it works.
Tested-by: Jiri Wiesner <jwiesner@suse.de>

[1] https://lore.kernel.org/lkml/20220617134325.GC30825@techsingularity.net/

--
Jiri Wiesner
SUSE Labs

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