Messages in this thread | | | Date | Mon, 16 Oct 2023 16:46:41 +0800 | Subject | Re: [PATCH v2 1/2] bus: mhi: host: Add spinlock to protect WP access when queueing TREs | From | Qiang Yu <> |
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On 9/29/2023 11:22 PM, Jeffrey Hugo wrote: > On 9/24/2023 9:10 PM, Qiang Yu wrote: >> >> On 9/22/2023 10:44 PM, Jeffrey Hugo wrote: >>> On 9/13/2023 2:47 AM, Qiang Yu wrote: >>>> From: Bhaumik Bhatt <bbhatt@codeaurora.org> >>>> >>>> Protect WP accesses such that multiple threads queueing buffers for >>>> incoming data do not race and access the same WP twice. Ensure read >>>> and >>>> write locks for the channel are not taken in succession by dropping >>>> the >>>> read lock from parse_xfer_event() such that a callback given to client >>>> can potentially queue buffers and acquire the write lock in that >>>> process. >>>> Any queueing of buffers should be done without channel read lock >>>> acquired >>>> as it can result in multiple locks and a soft lockup. >>>> >>>> Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org> >>>> Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com> >>>> --- >>>> drivers/bus/mhi/host/main.c | 11 ++++++++++- >>>> 1 file changed, 10 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c >>>> index dcf627b..13c4b89 100644 >>>> --- a/drivers/bus/mhi/host/main.c >>>> +++ b/drivers/bus/mhi/host/main.c >>>> @@ -642,6 +642,7 @@ static int parse_xfer_event(struct >>>> mhi_controller *mhi_cntrl, >>>> mhi_del_ring_element(mhi_cntrl, tre_ring); >>>> local_rp = tre_ring->rp; >>>> + read_unlock_bh(&mhi_chan->lock); >>> >>> This doesn't work due to the write_lock_irqsave(&mhi_chan->lock, >>> flags); on line 591. >> Write_lock_irqsave(&mhi_chan->lock, flags) is used in case of ev_code >> >= MHI_EV_CC_OOB. We only read_lock/read_unlock the mhi_chan while >> ev_code < MHI_EV_CC_OOB. > > Sorry. OOB != EOB > >>> >>> I really don't like that we are unlocking the mhi_chan while still >>> using it. It opens up a window where the mhi_chan state can be >>> updated between here and the client using the callback to queue a buf. >>> >>> Perhaps we need a new lock that just protects the wp, and needs to >>> be only grabbed while mhi_chan->lock is held? >> >> Since we have employed mhi_chan lock to protect the channel and what >> we are concerned here is that client may queue buf to a disabled or >> stopped channel, can we check channel state after getting >> mhi_chan->lock like line 595. >> >> We can add the check after getting write lock in mhi_gen_tre() and >> after getting read lock again here. > > I'm not sure that is sufficient. After you unlock to notify the > client, MHI is going to manipulate the packet count and runtime_pm > without the lock (648-652). It seems like that adds additional races > which won't be covered by the additional check you propose.
I don't think read_lock_bh(&mhi_chan->lock) can protect runtime_pm and the packet count here. Even if we do not unlock, mhi state and packet count can still be changed because we did not get pm_lock here, which is used in all mhi state transition function.
I also checked all places that mhi_chan->lock is grabbed, did not see packet count and runtime_pm be protected by write_lock(&mhi_chan->lock).
If you really don't like the unlock operation, we can also take a new lock. But I think we only need to add the new lock in two places, mhi_gen_tre and mhi_pm_m0_transition while mhi_chan->lock is held.
> >> >>> >>>> /* notify client */ >>>> mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result); >>>> @@ -667,6 +668,7 @@ static int parse_xfer_event(struct >>>> mhi_controller *mhi_cntrl, >>>> kfree(buf_info->cb_buf); >>>> } >>>> } >>>> + read_lock_bh(&mhi_chan->lock); >>>> } >>>> break; >>>> } /* CC_EOT */ >>>> @@ -1204,6 +1206,9 @@ int mhi_gen_tre(struct mhi_controller >>>> *mhi_cntrl, struct mhi_chan *mhi_chan, >>>> int eot, eob, chain, bei; >>>> int ret; >>>> + /* Protect accesses for reading and incrementing WP */ >>>> + write_lock_bh(&mhi_chan->lock); >>>> + >>>> buf_ring = &mhi_chan->buf_ring; >>>> tre_ring = &mhi_chan->tre_ring; >>>> @@ -1221,8 +1226,10 @@ int mhi_gen_tre(struct mhi_controller >>>> *mhi_cntrl, struct mhi_chan *mhi_chan, >>>> if (!info->pre_mapped) { >>>> ret = mhi_cntrl->map_single(mhi_cntrl, buf_info); >>>> - if (ret) >>>> + if (ret) { >>>> + write_unlock_bh(&mhi_chan->lock); >>>> return ret; >>>> + } >>>> } >>>> eob = !!(flags & MHI_EOB); >>>> @@ -1239,6 +1246,8 @@ int mhi_gen_tre(struct mhi_controller >>>> *mhi_cntrl, struct mhi_chan *mhi_chan, >>>> mhi_add_ring_element(mhi_cntrl, tre_ring); >>>> mhi_add_ring_element(mhi_cntrl, buf_ring); >>>> + write_unlock_bh(&mhi_chan->lock); >>>> + >>>> return 0; >>>> } >>> >
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