forked from Minki/linux
mainlining shenanigans
1f8da406a9
Tiny SRCU readers can appear at task level, but also in interrupt and softirq handlers. Because Tiny SRCU is selected only in kernels built with CONFIG_SMP=n and CONFIG_PREEMPTION=n, it is not possible for a grace period to start while there is a non-task-level SRCU reader executing. This means that it does not make sense for __srcu_read_unlock() to awaken the Tiny SRCU grace period, because that can only happen when the grace period is waiting for one value of ->srcu_idx and __srcu_read_unlock() is ending the last reader for some other value of ->srcu_idx. After all, any such wakeup will be redundant. Worse yet, in some cases, such wakeups generate lockdep splats: ====================================================== WARNING: possible circular locking dependency detected 5.15.0-rc1+ #3758 Not tainted ------------------------------------------------------ rcu_torture_rea/53 is trying to acquire lock: ffffffff9514e6a8 (srcu_ctl.srcu_wq.lock){..-.}-{2:2}, at: xa/0x30 but task is already holding lock: ffff95c642479d80 (&p->pi_lock){-.-.}-{2:2}, at: _extend+0x370/0x400 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&p->pi_lock){-.-.}-{2:2}: _raw_spin_lock_irqsave+0x2f/0x50 try_to_wake_up+0x50/0x580 swake_up_locked.part.7+0xe/0x30 swake_up_one+0x22/0x30 rcutorture_one_extend+0x1b6/0x400 rcu_torture_one_read+0x290/0x5d0 rcu_torture_timer+0x1a/0x70 call_timer_fn+0xa6/0x230 run_timer_softirq+0x493/0x4c0 __do_softirq+0xc0/0x371 irq_exit+0x73/0x90 sysvec_apic_timer_interrupt+0x63/0x80 asm_sysvec_apic_timer_interrupt+0x12/0x20 default_idle+0xb/0x10 default_idle_call+0x5e/0x170 do_idle+0x18a/0x1f0 cpu_startup_entry+0xa/0x10 start_kernel+0x678/0x69f secondary_startup_64_no_verify+0xc2/0xcb -> #0 (srcu_ctl.srcu_wq.lock){..-.}-{2:2}: __lock_acquire+0x130c/0x2440 lock_acquire+0xc2/0x270 _raw_spin_lock_irqsave+0x2f/0x50 swake_up_one+0xa/0x30 rcutorture_one_extend+0x387/0x400 rcu_torture_one_read+0x290/0x5d0 rcu_torture_reader+0xac/0x200 kthread+0x12d/0x150 ret_from_fork+0x22/0x30 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&p->pi_lock); lock(srcu_ctl.srcu_wq.lock); lock(&p->pi_lock); lock(srcu_ctl.srcu_wq.lock); *** DEADLOCK *** 1 lock held by rcu_torture_rea/53: #0: ffff95c642479d80 (&p->pi_lock){-.-.}-{2:2}, at: _extend+0x370/0x400 stack backtrace: CPU: 0 PID: 53 Comm: rcu_torture_rea Not tainted 5.15.0-rc1+ Hardware name: Red Hat KVM/RHEL-AV, BIOS e_el8.5.0+746+bbd5d70c 04/01/2014 Call Trace: check_noncircular+0xfe/0x110 ? find_held_lock+0x2d/0x90 __lock_acquire+0x130c/0x2440 lock_acquire+0xc2/0x270 ? swake_up_one+0xa/0x30 ? find_held_lock+0x72/0x90 _raw_spin_lock_irqsave+0x2f/0x50 ? swake_up_one+0xa/0x30 swake_up_one+0xa/0x30 rcutorture_one_extend+0x387/0x400 rcu_torture_one_read+0x290/0x5d0 rcu_torture_reader+0xac/0x200 ? rcutorture_oom_notify+0xf0/0xf0 ? __kthread_parkme+0x61/0x90 ? rcu_torture_one_read+0x5d0/0x5d0 kthread+0x12d/0x150 ? set_kthread_struct+0x40/0x40 ret_from_fork+0x22/0x30 This is a false positive because there is only one CPU, and both locks are raw (non-preemptible) spinlocks. However, it is worthwhile getting rid of the redundant wakeup, which has the side effect of breaking the theoretical deadlock cycle. This commit therefore eliminates the redundant wakeups. Signed-off-by: Paul E. McKenney <paulmck@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.