Re: [PATCH] random: remove batched entropy locking

["Jonathan Neuschäfer" <j.neuschaefer@gmx.net>]

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On Fri, Jan 28, 2022 at 04:33:44PM +0100, Jason A. Donenfeld wrote:
> From: Andy Lutomirski <luto@kernel.org>
> 
> We don't need spinlocks to protect batched entropy -- all we need
> is a little bit of care. This should fix up the following splat that
> Jonathan received with a PROVE_LOCKING=y/PROVE_RAW_LOCK_NESTING=y
> kernel:
> 
> [    2.500000] [ BUG: Invalid wait context ]
> [    2.500000] 5.17.0-rc1 #563 Not tainted
> [    2.500000] -----------------------------
> [    2.500000] swapper/1 is trying to lock:
> [    2.500000] c0b0e9cc (batched_entropy_u32.lock){....}-{3:3}, at: invalidate_batched_entropy+0x18/0x4c
> [    2.500000] other info that might help us debug this:
> [    2.500000] context-{2:2}
> [    2.500000] 3 locks held by swapper/1:
> [    2.500000]  #0: c0ae86ac (event_mutex){+.+.}-{4:4}, at: event_trace_init+0x4c/0xd8
> [    2.500000]  #1: c0ae81b8 (trace_event_sem){+.+.}-{4:4}, at: event_trace_init+0x68/0xd8
> [    2.500000]  #2: c19b05cc (&sb->s_type->i_mutex_key#2){+.+.}-{4:4}, at: start_creating+0x40/0xc4
> [    2.500000] stack backtrace:
> [    2.500000] CPU: 0 PID: 1 Comm: swapper Not tainted 5.17.0-rc1 #563
> [    2.500000] Hardware name: WPCM450 chip
> [    2.500000] [<c00100a8>] (unwind_backtrace) from [<c000db2c>] (show_stack+0x10/0x14)
> [    2.500000] [<c000db2c>] (show_stack) from [<c0054eec>] (__lock_acquire+0x3f0/0x189c)
> [    2.500000] [<c0054eec>] (__lock_acquire) from [<c0054478>] (lock_acquire+0x2b8/0x354)
> [    2.500000] [<c0054478>] (lock_acquire) from [<c0568028>] (_raw_spin_lock_irqsave+0x60/0x74)
> [    2.500000] [<c0568028>] (_raw_spin_lock_irqsave) from [<c030b6f4>] (invalidate_batched_entropy+0x18/0x4c)
> [    2.500000] [<c030b6f4>] (invalidate_batched_entropy) from [<c030e7fc>] (crng_fast_load+0xf0/0x110)
> [    2.500000] [<c030e7fc>] (crng_fast_load) from [<c030e954>] (add_interrupt_randomness+0x138/0x200)
> [    2.500000] [<c030e954>] (add_interrupt_randomness) from [<c0061b34>] (handle_irq_event_percpu+0x18/0x38)
> [    2.500000] [<c0061b34>] (handle_irq_event_percpu) from [<c0061b8c>] (handle_irq_event+0x38/0x5c)
> [    2.500000] [<c0061b8c>] (handle_irq_event) from [<c0065b28>] (handle_fasteoi_irq+0x9c/0x114)
> [    2.500000] [<c0065b28>] (handle_fasteoi_irq) from [<c0061178>] (handle_irq_desc+0x24/0x34)
> [    2.500000] [<c0061178>] (handle_irq_desc) from [<c056214c>] (generic_handle_arch_irq+0x28/0x3c)
> [    2.500000] [<c056214c>] (generic_handle_arch_irq) from [<c0008eb4>] (__irq_svc+0x54/0x80)
> [    2.500000] Exception stack(0xc1485d48 to 0xc1485d90)
> [    2.500000] 5d40:                   9780e804 00000001 c09413d4 200000d3 60000053 c016af54
> [    2.500000] 5d60: 00000000 c0afa5b8 c14194e0 c19a1d48 c0789ce0 00000000 c1490480 c1485d98
> [    2.500000] 5d80: c0168970 c0168984 20000053 ffffffff
> [    2.500000] [<c0008eb4>] (__irq_svc) from [<c0168984>] (read_seqbegin.constprop.0+0x6c/0x90)
> [    2.500000] [<c0168984>] (read_seqbegin.constprop.0) from [<c016af54>] (d_lookup+0x14/0x40)
> [    2.500000] [<c016af54>] (d_lookup) from [<c015cecc>] (lookup_dcache+0x18/0x50)
> [    2.500000] [<c015cecc>] (lookup_dcache) from [<c015d868>] (lookup_one_len+0x90/0xe0)
> [    2.500000] [<c015d868>] (lookup_one_len) from [<c01e33e4>] (start_creating+0x68/0xc4)
> [    2.500000] [<c01e33e4>] (start_creating) from [<c01e398c>] (tracefs_create_file+0x30/0x11c)
> [    2.500000] [<c01e398c>] (tracefs_create_file) from [<c00c42f8>] (trace_create_file+0x14/0x38)
> [    2.500000] [<c00c42f8>] (trace_create_file) from [<c00cc854>] (event_create_dir+0x310/0x420)
> [    2.500000] [<c00cc854>] (event_create_dir) from [<c00cc9d8>] (__trace_early_add_event_dirs+0x28/0x50)
> [    2.500000] [<c00cc9d8>] (__trace_early_add_event_dirs) from [<c07c8d64>] (event_trace_init+0x70/0xd8)
> [    2.500000] [<c07c8d64>] (event_trace_init) from [<c07c8560>] (tracer_init_tracefs+0x14/0x284)
> [    2.500000] [<c07c8560>] (tracer_init_tracefs) from [<c000a330>] (do_one_initcall+0xdc/0x288)
> [    2.500000] [<c000a330>] (do_one_initcall) from [<c07bd1e8>] (kernel_init_freeable+0x1c4/0x20c)
> [    2.500000] [<c07bd1e8>] (kernel_init_freeable) from [<c05629c0>] (kernel_init+0x10/0x110)
> [    2.500000] [<c05629c0>] (kernel_init) from [<c00084f8>] (ret_from_fork+0x14/0x3c)
> [    2.500000] Exception stack(0xc1485fb0 to 0xc1485ff8)
> [    2.500000] 5fa0:                                     00000000 00000000 00000000 00000000
> [    2.500000] 5fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
> [    2.500000] 5fe0: 00000000 00000000 00000000 00000000 00000013 00000000
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> [Jason: I extracted this from a larger in-progress series of Andy's that
>  also unifies the two batches into one and does other performance
>  things. Since that's still under development, but because we need this
>  part to fix the CONFIG_PROVE_RAW_LOCK_NESTING issue, I've extracted it
>  out and applied it to the current setup. This will also make it easier
>  to backport to old kernels that also need the fix. I've also amended
>  Andy's original commit message.]
> Reported-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
> Link: https://lore.kernel.org/lkml/YfMa0QgsjCVdRAvJ@latitude/
> Fixes: b7d5dc21072c ("random: add a spinlock_t to struct batched_entropy")
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Andy - could you take a look at this and let me know if it's still
> correct after I've ported it out of your series and into a standalone
> thing here? I'd prefer to hold off on moving forward on this until I
> receive our green light. I'm also still a bit uncertain about your NB:
> comment regarding the acceptable race. If you could elaborate on that
> argument, it might save me a few cycles with my thinking cap on.
> 
>  drivers/char/random.c | 57 ++++++++++++++++++++++++-------------------
>  1 file changed, 32 insertions(+), 25 deletions(-)

FWIW, this does fix the splat on my machine.

Tested-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>



Thanks everyone,
Jonathan

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