From: Michael Ellerman <mpe@ellerman•id.au>
To: Cyril Bur <cyrilbur@gmail•com>,
Balbir Singh <bsingharora@gmail•com>,
linuxppc-dev@lists•ozlabs.org, linux-mtd@lists•infradead.org
Cc: stewart@linux•vnet.ibm.com, alistair@popple•id.au,
dwmw2@infradead•org, rlippert@google•com
Subject: Re: [PATCH v3 06/10] powerpc/opal: Rework the opal-async interface
Date: Tue, 18 Jul 2017 13:20:08 +1000 [thread overview]
Message-ID: <8760eq1lef.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <1500338458.2475.3.camel@gmail.com>
Cyril Bur <cyrilbur@gmail•com> writes:
> On Mon, 2017-07-17 at 21:30 +1000, Balbir Singh wrote:
>> On Wed, 2017-07-12 at 14:23 +1000, Cyril Bur wrote:
>> > static int __opal_async_get_token(void)
>> > {
>> > unsigned long flags;
>> > int token;
>> >
>> > - spin_lock_irqsave(&opal_async_comp_lock, flags);
>> > - token = find_first_bit(opal_async_complete_map, opal_max_async_tokens);
>> > - if (token >= opal_max_async_tokens) {
>> > - token = -EBUSY;
>> > - goto out;
>> > - }
>> > -
>> > - if (__test_and_set_bit(token, opal_async_token_map)) {
>> > - token = -EBUSY;
>> > - goto out;
>> > + for (token = 0; token < opal_max_async_tokens; token++) {
>> > + spin_lock_irqsave(&opal_async_comp_lock, flags);
>>
>> Why is the spin lock inside the for loop? If the last token is free, the
>> number of times we'll take and release a lock is extensive, why are we
>> doing it this way?
>
> Otherwise we might hold the lock for quite some time.
No we won't. A loop over a few 10s or 100s of tokens is not going to
take long, compared to the overhead of taking the lock every iteration
through the loop.
If the linear search with the lock held is a bottle neck, then you can
keep a hint variable which tracks the last freed token and start
searching from there.
cheers
next prev parent reply other threads:[~2017-07-18 3:20 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-12 4:22 [PATCH v3 00/10] Allow opal-async waiters to get interrupted Cyril Bur
2017-07-12 4:22 ` [PATCH v3 01/10] mtd: powernv_flash: Use WARN_ON_ONCE() rather than BUG_ON() Cyril Bur
2017-07-17 6:19 ` Balbir Singh
2017-07-17 11:33 ` Frans Klaver
2017-07-18 0:27 ` Cyril Bur
2017-07-12 4:22 ` [PATCH v3 02/10] mtd: powernv_flash: Lock around concurrent access to OPAL Cyril Bur
2017-07-17 7:34 ` Balbir Singh
2017-07-17 7:55 ` Cyril Bur
2017-07-17 9:29 ` Balbir Singh
2017-07-18 1:14 ` Cyril Bur
2017-07-18 3:12 ` Michael Ellerman
2017-07-12 4:22 ` [PATCH v3 03/10] mtd: powernv_flash: Don't treat OPAL_SUCCESS as an error Cyril Bur
2017-07-17 8:50 ` Balbir Singh
2017-07-18 0:42 ` Cyril Bur
2017-07-12 4:22 ` [PATCH v3 04/10] mtd: powernv_flash: Remove pointless goto in driver init Cyril Bur
2017-07-12 4:22 ` [PATCH v3 05/10] powerpc/opal: Make __opal_async_{get, release}_token() static Cyril Bur
2017-07-12 4:23 ` [PATCH v3 06/10] powerpc/opal: Rework the opal-async interface Cyril Bur
2017-07-17 11:30 ` Balbir Singh
2017-07-18 0:40 ` Cyril Bur
2017-07-18 3:20 ` Michael Ellerman [this message]
2017-07-12 4:23 ` [PATCH v3 07/10] powernv/opal-sensor: remove not needed lock Cyril Bur
2017-07-12 4:23 ` [PATCH v3 08/10] powerpc/opal: Add opal_async_wait_response_interruptible() to opal-async Cyril Bur
2017-07-12 4:23 ` [PATCH v3 09/10] powerpc/powernv: Add OPAL_BUSY to opal_error_code() Cyril Bur
2017-07-12 4:23 ` [PATCH v3 10/10] mtd: powernv_flash: Use opal_async_wait_response_interruptible() Cyril Bur
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