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301913 |
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15-Jun-2016 |
sephe |
MFC 296178
buf_ring/drbr: Add buf_ring_peek_clear_sc and use it in drbr_peek
Unlike buf_ring_peek, it only supports single consumer mode, and it clears the cons_head if DEBUG_BUFRING/INVARIANTS is defined.
The normal use case of drbr_peek for network drivers is:
m = drbr_peek(br); err = hw_spec_encap(&m); /* could m_defrag/m_collapse */ (*) if (err) { if (m == NULL) drbr_advance(br); else drbr_putback(br, m); /* break the loop */ } drbr_advance(br);
The race is: If hw_spec_encap() m_defrag or m_collapse the mbuf, i.e. the old mbuf was freed, or like the Hyper-V's network driver, that transmission- done does not even require the TX lock; then on the other CPU at the (*) time, the freed mbuf could be recycled and being drbr_enqueue even before the current CPU had the chance to call drbr_{advance,putback}. This triggers a panic in drbr_enqueue duplicated element check, if DEBUG_BUFRING/INVARIANTS is defined.
Use buf_ring_peek_clear_sc() in drbr_peek() to fix the above race.
This change is a NO-OP, if neither DEBUG_BUFRING nor INVARIANTS are defined.
MFC after: 1 week Sponsored by: Microsoft OSTC Differential Revision: https://reviews.freebsd.org/D5416
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